Sorry to hear about your dead lamb
If it was rather empty, do you think Mum may have been struggling to feed it? And/or had mastitis or some reason she couldn't? Was it one of a pair or a single? How is Mum now?
If Mum wasn't by the lamb, perhaps she'd disowned it?
I have often found that with a ewe who has been reluctant to feed one or other of her lambs, so I brought her in and got it on her, kept her until she'd taken it, then put them out - that often, that lamb dies at a few weeks old. Sometimes I think there must have been something wrong with it, that the mother knew about, and was why she didn't want it in the first place. Sometimes I think she appears to be letting it suckle but is actually keeping it on short rations, so it isn't getting quite enough and eventually fails. Sometimes a lamb like this is seen suckling other ewes - a sure sign it's not getting enough from its own mother.